In a vehicle equipped with an internal combustion engine, a DPF is installed in an exhaust gas passage from the internal combustion engine to the atmosphere, and PM is caught by the DPF. The DPF is a member which temporarily catches the PM on a filter formed of porous ceramics and having a shape of a honeycomb pore.
If the amount of the PM caught by the DPF (hereinafter referred to as a PM deposit quantity) increases, the exhaust pressure of the internal combustion engine increases to lower the characteristics of the internal combustion engine, and therefore, processing for burning the caught PM is performed. This processing is referred to as DPF regeneration. In the DPF regeneration, fuel injection is performed to increase an exhaust temperature. When the exhaust temperature increases, the temperature of the DPF is increased, and the PM caught by the DPF is burned.
At that time, when the PM deposit quantity is too large, the DPF is damaged by heat generated in the DPF regeneration. In order to perform the DPF regeneration before the PM deposit quantity increases too much, the PM deposit quantity is required to be accurately detected.
As the PM sensor detecting the PM deposit quantity, there has been known a PM sensor in which two electrodes are installed in the DPF, and the PM deposit quantity is detected from a capacitance of a capacitor formed by the two electrodes. In this type of PM sensor, since the PM as a mixture of a dielectric with a conductor is deposited between electrodes, the capacitance increases linearly with respect to the PM deposit quantity.
In a conventional PM sensor 81 illustrated in FIG. 8, two electrodes 83 and 84 formed into a half-divided cylindrical shape are installed on an outer circumference of a columnar DPF 82. The two electrodes 83 and 84 face each other with the DPF 82 provided between the electrodes 83 and 84, whereby the capacitance of the capacitor formed by the two electrodes 83 and 84 is changed according to the PM deposit quantity of the entire DPF 82 (Patent Document 1).
In a conventional PM sensor 91 illustrated in FIG. 9, one electrode 93 is installed on an outer circumference of a columnar DPF 92, having a housing 95, and the other electrode 94 is concentrically installed inside the electrode 93. The capacitance of the capacitor formed by the two electrodes 93 and 94 is changed according to the PM deposit quantity of a portion of the DPF 92 (Patent Document 2).